1. Field of the Invention
The present invention generally relates to magnetic sensors and, more particularly, to a magnetic sensor for use with a position detector in which an electric resistance value changed by a relative movement of magnetoeffect resistance elements disposed in an opposing relation to a magnetic scale is detected and a relative position or the like is detected from the amount of the electric resistance value thus changed.
2. Description of the Prior Art
As is well-known, an electrical signal obtained on the basis of a relative movement of a magnetoeffect resistance element (hereinafter simply referred to as an MR element) disposed in an opposing relation to a magnetic scale magnetized at a constant grating pitch contains a harmonic component other than a fundamental wave component. It is desired that this harmonic component is eliminated because it deteriorates an accuracy in the interpolation processing.
A magnetic sensor 1 constructed as, for example, shown in FIG. 1 is proposed as a technique for reducing the harmonic component according to the prior art.
As shown in FIG. 1, in the magnetic sensor 1, MR elements 5, 6 opposed to a magnetic scale 2 with a grating pitch .lambda. and which are distant from MR elements 3, 4, each of which is distant by .lambda./4, by a distance .lambda./12 are formed on a substrate (not shown) so as to eliminate a third-order harmonic component and the magnetic sensor 1 is connected between a power supply source VO and the ground. An output V1 is generated from a junction between the MR elements 5 and 4. The output V1 is applied to an amplifier 10 formed of resistors 8, 9 and an operational amplifier 11 as shown in FIG. 2 and a sine wave signal is obtained from an output V2 of the amplifier 10 when the magnetic sensor 1 is moved relatively in the length direction of the magnetic scale 2. A reference voltage VR is supplied to a non-inverting input terminal of the operational amplifier 11.
In the industrial field in which the length is measured by using the magnetic scale 2, e.g., in the machine tool field, it is required that the magnetic sensor becomes higher in accuracy. In order to make the magnetic sensor become higher in accuracy, the grating pitch of the magnetic scale 2 must be reduced. However, in the magnetic sensor 1 constructed as shown in FIG. 1, a pattern width W1 of the MR elements 3 through 6 is about 10 .mu.m at best because of various limits such as sensitivity, manufacturing technique of MR elements or the like. Further, a clearance D1 between pattern edges of adjacent MR elements (e.g., the MR elements 3 and 5) is about 5 .mu.m at best. Accordingly, as to the length of .lambda./12, the minimum value (.lambda./12) min thereof is represented as (.lambda./12) min=W1+D1=15 .mu.m. As a consequence, the minimum value .mu.min of the grating pitch .lambda. cannot be reduced to less than .mu.min=12.times.(W1+D1)=180 .mu.m.
The assignee of the present application has previously proposed a technique which can solve the aforesaid problem (see Japanese Patent Application No. 2-150688). This previously-proposed magnetic sensor technique will be described with reference to FIG. 3.
As shown in FIG. 3, an MR element 12 or 13 having a crank-shaped portion is formed as a magnetic sensor element. The pattern width of the crank-shaped portion is either increased as compared with that of its straight portion or is formed of a conductive material. In the example of FIG. 3, since a clearance D2 corresponding to the clearance D1 shown in FIG. 1 may be reduced to zero (D2=0), there is then the excellent advantage such that the grating pitch .lambda. can be reduced.
In the magnetic sensor shown in the example of FIG. 3, when the length direction of the magnetic scale 2 and the length direction of the pattern of the magnetic sensor element are displaced by an azimuth angle .theta.1, then a position at which the harmonic wave involved in the output V1 is minimized and a position at which the output V1 itself is maximized ar displaced from each other as shown in FIGS. 4A and 4B. The reason for this is that, when the magnetic sensor is disposed with an inclination angle as shown in FIGS. 4A and 4B, then the lateral magnetic field H acts on the entirety of the MR elements 12, 13, resulting in the resistance thereof being changed considerably.
In actual practice, MR elements 15 to 18 constitute a magnetic sensor 14 as shown in FIG. 5, the MR elements 15 to 18 of this magnetic sensor 14 constitute a bridge circuit and a differential amplifier 24 formed of resistors 19 to 22 and an operational amplifier 23 is connected to the bridge circuit as shown in FIG. 6. FIG. 7 shows an output characteristic of the circuit of FIG. 6. As is clear from FIG. 7, a position (.theta.=-10') at which an output V5 [V] of the circuit of FIG. 6 is maximized and a position (.theta.=-30') at which a harmonic distortion N/S [dB] such as three-order harmonic distortion D3A, five-order harmonic distortion D5A or the like is minimized are displaced from each other relative to the change of the azimuth angle .theta. (minute).
When the disposed position of the magnetic sensor 14 or the like relative to the magnetic scale 12 is determined, then the disposed position is determined by adjusting a clearance between a head holder (not shown) in which the magnetic sensor 14 is housed and the magnetic scale 2, the azimuth angle or the like. When the clearance, the azimuth angle or the like are adjusted in order to determined the disposed position, it is easy and reliable to perform the adjustment in such a fashion that the amplitude of the output V5 may be maximized while a magnitude of the output V5 is observed by means of an oscilloscope or the like.
However, when the clearance, the azimuth angle or the like are adjusted as described above, the position (azimuth angle .theta.) at which the maximum value of the output V5 is obtained and the position (azimuth angle .theta.) at which the minimum value of the harmonic wave (three-order harmonic distortion D3A, five-order harmonic distortion D5A or the like) are not coincident with each other. As a result, when the output V5 is processed by an interpolation processing in order to make the measuring become higher in accuracy, there is then the new problem such that the measuring at high accuracy is made rather difficult by the influence of the harmonic distortion.